1. Field of the Invention
The present invention relates to image forming devices, and more particularly to those image forming devices having a redundant electrical contact between a power supply and a photoconductive drum.
2. Description of the Related Art
Image forming devices include copiers, laser printers, facsimile machines, and the like. Image forming devices may have a photoconductive drum having a photoconductive surface that is typically charged to a uniform electrical potential. The photoconductive surface is selectively exposed to light in a pattern corresponding to an original image. As a result of this selective exposure, certain areas of the photoconductive surface are discharged resulting in formation of a latent electrostatic image thereon.
A developer material, such as toner, having an opposite electrical charge relative to the photoconductive drum, when brought into contact with or otherwise near the photoconductive surface becomes attracted to the photoconductive surface due to the charge placed thereon. Further, a recording medium, such as a blank sheet of paper or an intermediate transfer belt, is then brought into physical contact with the photoconductive surface. This physical contact allows the developer material to be transferred to the recording medium in the form of the latent electrostatic image when subjected to an electric field. Once the developer material is transferred to the sheet of paper, the sheet is then heated thereby permanently fusing the toner thereto.
As illustrated in FIG. 1, image forming devices typically include a printer supply contact 10 that is coupled to a power supply. The printer supply contact 10 is electrically coupled to a bearing 12 which in turn is electrically connected to an end of a shaft 14. The shaft 14 is received within the photoconductive drum 18 and electrically coupled thereto via a photoconductive drum contact 16. As a result of this electrical connectivity, photoconductive drum 18 may be charged to the desired level to support image transfer.
Thus, electrical connectivity of the bearing 12 with respect to the shaft 14 and the power supply contact 10 serves to ensure substantially uninterrupted flow of electric charge between the printer supply contact 10 and the shaft 14. Moreover, as the bearing 12 physically supports the shaft 14 and also a substantial load of a printer cartridge, bearing 12 should provide enough rigidity and strength to support the printer cartridge.
However, these characteristics have not always been sufficiently shown with conventional conductive bearings. This is because some conductive bearings are made from materials such as sintered bronze or conductive plastic, which are seen to have disadvantages.
One disadvantage of sinter bronze bearings is that such bearings contain oil that may be expelled when higher temperatures are experienced during prolonged imaging operations. This expelled oil collects debris, such as toner or paper dust, from the image forming device and coats the shaft of the photoconductive drum. This debris coating causes the electrical contact between the shaft and the bearing to fail, thereby preventing the photoconductive drum to be suitably charged. Another disadvantage is that the conductive plastic bearings may have a non-conductive outer layer that have been seen to interrupt the flow of electric charge between the bearing and the shaft.
Thus, given the above drawbacks of the conventional systems for charging photoconductive drums, there is a need for a charging system which provides substantially continuously supply of electric charge to the photoconductive drum of an image forming device.